Sensors for detecting radiation are useful in many fields and applications. One of those fields is spectroscopy. Spectroscopy is a scientific technique by which electromagnetic radiation from a given source is broken down into its wavelength components and those components are analyzed to determine physical properties of the source of that radiation. Particularly, the wavelengths of radiation that are or are not in the spectrum are indicative of the atoms or molecules that are in the source of the radiation. The term “source” herein encompasses not only objects that generate radiation, but also objects that merely reflect, transmit, or absorb radiation emitted by another source.
Spectroscopy and spectrometers have powerful and important applications in many fields of science and technology. For example, spectroscopy and spectrometers are used extensively in astronomy to determine the composition of stars and other objects in space. Spectroscopy and spectrometers also are used in military and security applications, such as in the identification of substances that might be inside of buildings, underground, concealed on persons or otherwise not directly observable. Spectrometers also can be used to scan persons and luggage (at airports, for instance) to determine if the person is carrying (or the luggage contains) certain types of items, such as plastic explosives or metal objects, such as firearms.
U.S. patent application Ser. No. 11/305,541 entitled Spectrometer Method and Apparatus For Near Infrared to Terahertz Wavelengths filed Feb. 9, 2006 discloses an exemplary spectrometer architecture that can be adapted for use across a very wide spectrum of bandwidths, and is incorporated fully herein by reference. Such spectrometers employ one or more sensors for detecting the radiation that is received in the spectrometer.
Within these spectra, one can study emission and/or absorption lines, which are the fingerprints of atoms and molecules. Every atomic element in the periodic table of elements has a unique spacing of electron orbits and, therefore, can emit or absorb only certain energies or wavelengths. Thus, the location and spacing of spectral lines is unique for each atom and, therefore, enables scientists to determine what types of atoms are within a radiation source from its unique signature spectrum.
Spectroscopy based on atomic spectral lines is primarily appropriate for visible wavelengths. In the near infrared (IR) range (which is roughly 0.75-3.0 microns), midwave IR range (about 3.0-8.0 microns), and longwave IR range (about 8.0-30 microns), the dominant mechanism responsible for spectral absorption bands are not transitions between electronic energy levels, but rather transitions between molecular vibrational energy levels. In the far IR range, sometimes referred to as the Terahertz or THz range (about 30-3000 microns), molecular rotational energy levels are the dominant mechanism.
Primarily in the THz regime (far IR), there is an even further physical mechanism that spectroscopy can be used to detect. Specifically, solid materials exhibit different spectra based on the absorption spectra of the material's crystalline lattice vibrations (so called phonon spectrum), which lie mostly at far IR wavelengths (THz frequencies). The principle is the same, but the fundamental mechanism for spectral emissions is lattice vibrations rather than molecular vibrations or rotations. This is useful for detecting explosives, drugs, etc.
Even further, continuous spectra (also called thermal spectra) are emitted by any object that radiates heat, i.e., has a temperature above absolute zero. The light (or other electromagnetic radiation) is spread out into a continuous band with every wavelength having some amount of radiation. Accordingly, the magnitude of radiation at a given wavelength or wavelengths may be used to determine the general composition of an object and/or its temperature or density.
A non-imaging spectrometer observes the spectral components of all the radiation from a given source as a single unit. On the other hand, an imaging spectrometer separately detects the radiation from different points in a given field of view and determines the spectral components for each of those points separately (i.e., pixelation). Thus, for instance, a non-imaging spectrometer may employ a single sensor for detecting the radiation from an object, whereas an imaging spectrometer would comprise an array of sensors and some optical apparatus to guide the radiation into the array such that each sensor receives radiation from a different portion or point within the overall field of view being observed.
In any event, spectrometers as well as other types of scientific, industrial and military equipment utilize radiation sensors for detecting radiation.
U.S. Pat. Nos. 5,220,188 and 5,220,189 disclose one particular micromechanical thermoelectric sensor element that can be used in spectroscopy for detecting radiation. In such a design, a micromechanical structure is suspended over a pit etched into a silicon substrate. A thermocouple is formed in the suspended structure which produces a voltage in response to a temperature change, which voltage can be provided to read out electronics so as to detect the presence of radiation (which was the cause of the temperature change).
There are many types of sensor designs that can be used to detect radiation in a spectrometer or other instrument. The particular technology most suitable for a particular application depends on the frequency range of the spectrometer or other instrument, different technologies being more economically suited to different wavelengths of radiation. In the near infrared wavelength range, the detector might comprise charge coupled devices, such as photoelectric detectors using either the photoconductive effect or photovoltaic effect, fabricated using MEMS technology. In the Terahertz range (roughly 0.1 THz to 20 THz), on the other hand, a thermal detector, such as a thermoelectric (TE) microbridge detector would be a more suitable choice as a sensor. Such microbridge detectors can be manufactured using MEMS technology. Some particular TE microbridge detectors are disclosed in U.S. Pat. Nos. 5,220,188, 5,220,189, 5,449,910, and 6,036,872, owned by the same assignee as the present patent application. U.S. Pat. No. 5,220,188 discloses a basic etch-pit type of microbolometer IR detector. U.S. Pat. No. 5,220,189 discloses a basic thermoelectric (TE) type IR detector. Subsequent improvements to these designs are described in, for instance, U.S. Pat. Nos. 5,449,910, 5,534,111, 5,895,233, and 6,036,872. Also, Golay cells are known for use in sensing in the THz range.
The U.S. government and other organizations are interested in the development of spectrometers that can operate in the THz regime with very high sensitivity, e.g., having a signal to noise ratio of 1:1 or better at 1 pico-watt per root Hertz of electrical bandwidth, pW/vHz. As previously noted, spectroscopy in the THz regime can be useful for detecting concealed drugs and explosives. Therefore, there is a need for an extremely sensitive and reliable radiation detector that can operate in the THz regime.
Therefore, it is an object of the present invention to provide an improved radiation sensor.